Irradiation of condensed meiotic chromosomes in lilium longiflorum

Conclusions

The following X-ray studies have been conducted on the microsporocyte chromosomes of Lilium longiflorum:

• 1. Chromosomes irradiated at late diakinesis or MI and examined 2 hours later at AI and also 16–19 hours later at AII.

• 2. Chromosomes irradiated at MII and examined 2 hours later at AII.

• 3. Chromosomes irradiated at MI following varying intervals of pretreatment with chloramphenicol.

  The following conclusions have been reached:

• 1. Chromosomes irradiated at MI and MII respectively yield only halfchromatid aberrations 2 hours later at AI or at AII. The frequency of aberrations per chromatid is approximately the same in both cases.

• 2. The frequency of half-chromatid aberrations induced at MI and scored at AI shows a linear relationship to X-ray dose.

• 3. Chromosomes irradiated at MI and examined at AII yield acentric fragments and chromatid bridges in addition to half-chromatid bridges. Because of the high number of chromosomes and the small size of the second meiotic spindle in lily, analysis at AII is far from satisfactory. The cells scored at AII are very few and the data subject to considerable error. Nevertheless, the high frequency of acentrics found at AII supports the view that X-rays can break both sister half-chromatids at the same locus at MI; the acentric fragment is not released until telophase. Moreover, the data suggest that the probability of breakage of two sister half-chromatids at the same locus exceeds the probability of breakage of two non-sister half-chromatids.p ]The frequency of half-chromatid bridges observed at AII, following irradiation at late diakinesis or MI, is lower than expectancy.

• 4. The chloramphenicol treatments applied did not alter the frequency of half-chromatid bridges at AI following irradiation at MI.